This invention relates generally to machines that bring a tool rapidly to a workpiece and then apply axial force to the workpiece. Such tools include taps, buffers, sanders, drills, grinders, millers, honers, lathes and the like. More specifically, the invention relates to an apparatus for tapping holes creating a helical threadform of predetermined depth in high volume production operations. Specifically, the invention relates to a hole tapping apparatus having a small number of moving parts and a fluid controlled actuator and control system.
Known apparatus for tapping holes in high volume production operations are generally lacking in many respects. Both the speed of the apparatus and the means for controlling cutting conditions require improvement.
Tapping machines engage a tap, and feed it into the work. In ideal conditions of a perfectly fabricated and uniformly sharp tap, the leading cutting surfaces of the tap cut a helical thread in the workpiece. The thread exactly matches the pitch and size of the trailing cutting surfaces of the tap. Thus, the trailing edges smoothly travel along the helical threads already created by the leading surfaces. The helical thread cut by the leading surfaces is referred to as the "lead." However, the ideal condition rarely, if ever, exists. Taps are not cut perfectly, and do not wear uniformly. Consequently, the typical condition is that the leading surfaces of the tap will tend to cut a helical thread lead of a certain shape while the trailing edges tend to cut a thread form of a differing shape. If only minimal axial force is applied to the tap (e.g., enough to engage the cutting edge but no more), the tap will draw itself into the hole at a rate dictated by the lead that it itself generates based on the pitch of the tap. Such would occur in the case of a hand tapping operation.
Automatic and power tapping machines, however, usually have a heavy lead screw to feed the tap to and into the work. Consequently, the lead screw tends to feed the tap into the work at a given rate dictated by the pitch of the lead screw. The phenomenon of the tap tending to generate one lead pattern and the lead screw tending to generate a different pattern is referred to as "redundancy". In any case where redundancy exists, the error, (i.e. the difference between the ideal path that would be generated by an ideal tap driven by the lead screw and the path that the real tap would generate on its own) increases as the length of the thread increases.
If the difference between the ideal path and the actual path are large, the tap will bind in the hole being formed. Binding itself is a serious problem. However, in cases where the driving lead screw is not adapted to discontinue applying axial force, the tap or the threads being generated may be damaged or broken.
In order to avoid the redundancy problem, conventional tapping machines allow the tap to "float" axially along the direction that the tap travels. Other mechanisms to relieve the "redundancy" involve making the linkage between the lead screw and the tap sufficiently flexible to allow for both rotational and axial spring motion.
Another drawback of conventional tapping apparatus and other shaping tools in general relates to high speed production. It is desirable that the tool be: brought to the workpiece; driven in a forward motion into the workpiece to shape the workpiece; driven out of the workpiece and withdrawn to the rest position, all as quickly as possible.
Tapping machines generally employ the lead screw to bring the tap to the workpiece. Some lead screws cause the tap to revolve at a constant speed, the cutting speed, as the screw advances. This results in a wasteful compromise. On the one hand, it is desirable to bring the tap to the workpiece as quickly as possible. For a given lead screw pitch, this would require a high RPM. On the other hand, once introduced into the workpiece, the tap has a maximum cutting speed, above which it will break or jam. Thus, for cutting speed purposes, a lower speed is desirable. Some machines have a control mechanism which senses the proximity of the tap to the workpiece, and adjusts the rotational speed accordingly. Such control mechanisms are unduly expensive.
Further, most shaping machines in use incorporate in their drive train large and heavy inertial bodies, including the lead screw and gearing to gear the rotary action of the tool up or down from the rotational speed of the driving engine. Consequently, at the point of rotation reversal, a large force must be applied to overcome and reverse the forward momentum. These machines tend to wear out relatively quickly due to the forces and attendant wear generated upon reversal. They are also quite noisy and are prone to vibration. The problem is particularly acute with machines for driving small tools, which must run at very high RPM's. The speed is generally obtained using a rotary motor operating at a high revolutionary speed, which itself must be reversed as quickly as possible.
Tapping machines encounter the further complication that in order to remove the tap from the threaded hole, its rotation must be reversed.
An additional problem of known machines and tapping machines in particular is that they incorporate a large number of moving parts which increases the original cost and the likelihood of misalignment, misfitting, wear and need for maintenance.
Yet another drawback of the apparatus of the prior art is that they require a relatively complicated system of sensors and feedback elements in order to automatically advance to the workpiece, shape the workpiece, e.g. tap the hole, reverse direction and retract from the workpiece, etc. Tapping machines require a particularly complicated control system, due to the required rotation reversal and speed change. Control systems of prior art tapping machines are inadequate or particularly costly. Various and often different types of sensors are required to determine when the tool has contacted the workpiece; when the tool has shaped the piece as desired; and when the tool has cleared the workpiece upon retraction.
Prior art tapping machines also cannot easily reverse the sense (i.e. right-handedness or left-handedness) of the threaded hole they are tapping, due to the fact that they are typically driven directly by a rotary motor through a gear train. It is necessary to interpose a reversing gear set in order to provide threaded holes of both hands.
Additionally, prior art machines each have a characteristic stall torque, i.e., a torque applied at the tool which will seize-up the driving mechanism. If the stall torque of the driving mechanism is greater than the torque that will damage the tool, then the tool will frequently be damaged. It is desirable to be able to adjust the stall torque, depending upon the requirements of the tool.
Other problems with such machine tools of the prior art, including tapping machines are that they are energy inefficient and heat up at high cycling rates and dissipate power even during standby conditions. They typically require electrical power, rather than safe and commonly available shop air pressure. They are typically costly, both to manufacture and to maintain.